Taste sensation Scientists have pinpointed 'hotspots' in the brains of mice that respond to each of the known taste senses, except for one - sour.

The research, led by Professor Charles Zuker and Dr Xiaoke Chen of Columbia University in the United States, suggests that there is a similar map for taste in the brain, just as there are maps for vision and hearing.

Although we experience foods as having a wide variety of flavours, there are thought to be only five tastes detected by cells on the tongue: sweet, salty, bitter, sour and umami.

A sense of taste has important survival implications, say the researchers. The umami taste receptor, which responds to monosodium glutamate and other amino acids, probably exists to detect valuable protein-rich foods. The bitter taste warns against ingesting poisons.

"Over the past 10 to 15 years we have indentified [tongue] receptors for sweet, bitter, salt and sour" says Dr Nicholas Ryba of the National Institute of Health in Bethesda, USA, a member of the team.

He explains that so far their findings all fit a 'one cell-one taste' paradigm. For example, one tongue cell may detect bitter substances, whilst its neighbour detects salt.

In this study, the scientists wondered whether tastes are represented separately in the brain too. The gustatory cortex, which registers taste, is tiny and tucked away in a part of the brain called the insula.

They examined the insulas of anaesthetised mice using a technique known as two-photon calcium imaging.

"It's a relatively new type of microscopy that uses a quantum effect", says Ryba, "which enables you to image more deeply within a tissue". In this case "more deeply" means 0.2 millimetre into the cortex.

"When a nerve cell is activated, there's a wave of calcium in the cell", says Ryba.

The team loaded up brain cells with a calcium-sensitive fluorescent dye, so when a nerve cell fired they could see it as a burst of fluorescence. Using this technique they were able to look at hundreds of neurons at a time and see whether they were responding.

More than a taste sensation

By putting different-tasting substances on the tongues of the mice, and watching for responses in the brain cells, they mapped out "four hotspots that are completely separate" containing cells responding to sweet, bitter, umami and salty tastes.

"It's a little bit mysterious why we didn't find sour", says Ryba. "Maybe it is separated somewhere quite far away from where we were looking."

"Sour does have components that are probably not taste", he adds and points out that sour substances can stimulate pain receptors. "Think of the pain when lemon juice gets in your eye, for instance".